The present invention relates generally to voltage suppression networks and more particularly to a circuit for suppressing the voltage coupled to a motor or generator shaft to a value sufficient to eliminate the arcing between the shaft and its bearings.
One typical motor or generator structure includes a field winding mounted on a shaft which is moveable, with respect to the stationary portions of the machine, within suitable bearings. Although the field winding is electrically insulated from the shaft, there are, nonetheless, voltages which are capacitively coupled to the shaft from the energized winding. In high voltage machines (e.g., 460 volts and above) this coupled voltage can be relatively high, for example 80 volts or more, such that arcing between the shaft and the bearings can result in pitting of the shaft and/or bearings, thus, resulting in early motor failure. It has been found that voltages as low as eight volts may, in certain instances, result in such arcing and pitting.
It has long been known in the art to provide what is known as a shaft voltage suppressor network or circuit. One form of the shaft voltage suppressor includes two legs, each including the series combination of a resistor and a capacitor, which two legs are also connected in series with the total series combination being connected in parallel with the field winding. The common point of the two legs is connected to a common voltage point such as earth ground. Such shaft voltage suppression networks function quite adequately in those situations where the field is supplied from a balanced source such as the well known six-thryristor rectifying bridge connected to a three-phase line voltage. When, however, a hybrid rectifying bridge, for example one including three controlled rectifiers (e.g., thyristors) and three uncontrolled rectifiers (e.g., diodes) is used, the non-symmetry of the bridge causes different field voltage waveforms. In this situation the known shaft voltage suppression circuit such as that described above is incapable of appropriately reducing the voltage coupled to the shaft such that once again arcing and the pitting of the component results.